1. Field of the Invention
The present invention relates to novel polymers having improved resistance to heat, and, more especially, to novel polymers comprising maleimido recurring structural units.
2. Description of the Prior Art
The polymerization of N-substituted maleimides by free radical and anionic routes is known to this art. Thus, R, Cubbon, Polymer, 6, 419 (1965) has reported that polymers having a predominantly threodiisotactic configuration are obtained by polymerizing N-ethylmaleimide using n-butyllithium as initiator:
(i) either in toluene, at temperatures not exceeding -20.degree. C., for period of time ranging from 20 minutes to 3 hours, providing a yield not exceeding 22%; or
(ii) in tetrahydrofuran, at -70.degree. C., rapidly providing a yield ranging up to 95%.
With regard to N-phenylmaleimide, T. Hagiwara et al, Makromol. Chem., Rapid Commun., 6, 169 (1985) has described the influence of the selection of the initiator and the reaction conditions on the anionic polymerization and has confirmed the observations of R. Cubbon and also shown that:
(i) yields in excess of 85% are obtained in tetrahydrofuran between -72.degree. C. and O.degree. C. for reaction periods of 30 to 60 minutes in the presence of potassium tert-butanolate. The number-average molecular weight of the polymer obtained can be as high as 8,000 for a reaction period of 3 hours at -72.degree. C.; and
(ii) a quantitative yield in the presence of a large proportion of lithium tert-butanolate can be obtained at O.degree. C. in tetrahydrofuran, the resulting polymer having a number-average molecular weight of only 2,000.
As regards N-ethylmaleimide, the above observations have also been confirmed by T. Hagiwara et al, in J. Polym. Sci., Polym. Chem. Ed., 26, 1011 (1988). However, in addition to the above confirmation, the authors present the possibility of excellent polymerization yields at +24.degree. C. in the presence of potassium tert-butanolate, equally well in toluene as in tetrahydrofuran, the active nature of the reaction process also permitting a number-average molecular weight of up to 6,400 to be attained.
From the above prior art, it will be seen, in summary, that, on the one hand, the polymerization of N-substituted maleimides by an anionic route produces results which are highly differentiated depending on the nature, alkyl or aryl, of the N-substitution and, with a single exception, depending on the nature of the solvent used. On the other hand, to date no process has been proposed to the art enabling polymers of high molecular weight to be produced (the attainable degree of polymerization appearing to be limited to about 50). From the first observation, it will be appreciated that need exists in this art for polymers having a controlled, and preferably narrow molecular weight distribution, prepared by a process that proceeds under industrially safe conditions. From the second observation, it will be appreciated that polymers having too low a molecular weight have scant industrial applications.